It is a known characteristic of a number of microorganisms, that they possess constituents which can be assembled in an orderly manner from one compound, often of low molecular weight, to provide a high molecular weight entity. These high molecular weight entities may take various forms, for example, as protein coats in the case of viruses, as pili in the case of certain bacteria and when the low molecular weight material is a carbohydrate the high molecular weight material can be the cell wall of a bacteria. This high molecular weight material is often highly immunogenic.
As an example of this assembly process, the Hepatitis B surface antigen is composed of a glycoprotein of molecular weight 29,000 Daltons, which on aggregation forms a particle containing 130 of the sub-units, having a molecular weight of 3.8.times.10.sup.6 Daltons and known as the Dane particle. A similar situation occurs with Tobacco Mosaic Virus (hereinafter abbreviated to TMV) where a smaller protein aggregates in a single helix to form rods, containing large numbers of the smaller protein. Often, in the case of viruses, the sub-unit proteins can aggregate to their final polymeric form without the other constituents of the virus being present and under completely in vitro conditions.
It is also known that small molecules, which often act only poorly as immunogens in their ability to elicit antibodies in an in vivo system, when attached to a larger molecule that is itself antigenic, will give rise to improved antibody response to the smaller molecule, as well as the normal response to the larger carrier molecule. The small molecule attached to the larger in this system , is called a hapten, and can vary in size from small to quite large. In one example of this combination, of interest to the health care field, a small portion of the Hepatitis B surface antigen, comprising a sequence of fifteen amino acids, which is not itself antigenic, has been covalently bound to the antigenic protein, keyhole limpet hemocyanin, and the resulting conjugate elicited antibodies in an in vivo system that cross-reacted with the native surface antigen and also with the whole hepatitis virus. This system of carrier-hapten could be the basis for an effective vaccine against a disease for which the hapten codes.
In a modification of this general idea as disclosed in U.S. Pat. No. 4,496,538 a carrier hapten vaccine may comprise a hapten which is a carbohydrate from a known pathogenic organism, Haemophilus influenzae b, and a carrier which is diphtheria toxoid. It has been shown that this carrier hapten is not only immunogenic but will protect against the disease.
Recombinant DNA techniques have been used to produce fairly large quantities of proteins in microorganisms, that are normally foreign to the organism. Such techniques involve inserting into the organism a vector containing DNA coding for the desired foreign protein. In this way it is possible for the organism to produce not only foreign proteins, but also proteins that differ from the naturally occurring ones by having either changed amino acid sequences, additions or subtractions. The gene coding for these foreign proteins may be isolated from a natural source, may be synthesized, and may be altered using restriction enzymes to obtain the gene for a changed protein.
Such recombinant DNA techniques, applied to the formation of viral proteins, permit the preparation of proteins, which can be used as vaccines, which would contain none of the contaminants associated with present viral vaccines produced by standard methods, where the virus is grown on a cell substrate and the final vaccine can contain products from the cell substrate, the medium and nucleic acid from the virus, all of which can have deleterious effects.